Programmable logic devices exist as a well-known type of integrated circuit (IC) that may be programmed by a user to perform specified logic functions. There are different types of programmable logic devices, such as programmable logic arrays (PLAs) and complex programmable logic devices (CPLDs). One type of programmable logic devices, called the field programmable gate array (FPGA), is very popular because of a superior combination of capacity, flexibility and cost. A FPGA typically includes an array of configurable logic blocks (CLBs) surrounded by a ring of programmable input/output blocks (IOBs). The CLBs and IOBs are interconnected by a programmable interconnect structure. The CLBs, IOBs, and interconnect structure are typically programmed by loading a stream of configuration data (bitstream) into internal configuration memory cells that define how the CLBs, IOBs, and interconnect structure are configured. The configuration bitstream may be read from an external memory (e.g., an external PROM). The collective states of the individual memory cells then determine the function of the FPGA.
Due to advances in semiconductor processing technology, more and more transistors can be fabricated onto the same area in an IC. This leads to more functionality. As a result, pin counts of the devices need to be increased to support the functionality. Recently, some of the FPGAs have around one thousand pins.
Because these FPGAs can be programmed to perform many functions, they are used in more and more product designs. In some complex product designs, more than one FPGA is used in a product. Some of these FPGAs need to start operation at different times after configuration. In the past, engineers have to design glue logic to handle the configuration and start time of these FPGAs. In many cases, this glue logic takes up valuable real estate on a circuit board. In addition, the glue logic is typically custom designed for each product. Consequently, it is a time consuming and inefficient process.
The large number of pins on a FPGA also means that the circuit board is more congested because many of the pins are connected to other ICs. Thus, it is increasing difficult to find space on a circuit board to place the above-mentioned glue logic.
Therefore, it is desirable to reduce unnecessary circuits on a circuit board. It is also desirable to improve efficiency in using FPGAs.